Semiconductor device and method for manufacturing the same

ABSTRACT

It is an object to provide an oxide semiconductor which is suitable for use in a semiconductor device. Alternatively, it is another object to provide a semiconductor device using the oxide semiconductor. Provided is a semiconductor device including an In—Ga—Zn—O based oxide semiconductor layer in a channel formation region of a transistor. In the semiconductor device, the In—Ga—Zn—O based oxide semiconductor layer has a structure in which crystal grains represented by InGaO 3 (ZnO) m  (m=1) are included in an amorphous structure represented by InGaO 3 (ZnO) m  (m&gt;0).

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a semiconductor device using an oxidesemiconductor and a method for manufacturing the semiconductor device.

2. Description of the Related Art

A field-effect transistor (also referred to as an FET) is the mostwidely-used semiconductor element at present. A variety of materials areused for field-effect transistors according to their uses. Inparticular, semiconductor materials including silicon are frequentlyused.

The field-effect transistor using silicon has characteristics satisfyingthe needs with respect to a variety of uses. For example, single crystalsilicon is used for an integrated circuit or the like which needs to beoperated at a high speed, whereby the need with respect to theintegrated circuit is satisfied. Further, amorphous silicon is used foran object which needs a large area, such as a display device, wherebythe need with respect to the object can be satisfied.

As described above, silicon is highly versatile and can be used forvarious purposes. However, in recent years, semiconductor materials tendto be expected to have higher performance as well as versatility. Forexample, in terms of improving performance of a large-area displaydevice, in order to realize high-speed operation of a switching element,a semiconductor material which facilitates increase of the area of adisplay device and has higher performance than amorphous silicon isneeded.

Under such conditions, a technique relating to a field-effect transistorusing an oxide semiconductor has attracted attention. For example, inPatent Document 1, a transparent thin film field-effect transistor usinga homologous compound InMO₃(ZnO)_(m) (M is In, Fe, Ga, or Al, and m isan integer number of greater than or equal to 1 and less than 50) isdisclosed.

In addition, in Patent Document 2, a field-effect transistor isdisclosed in which an amorphous oxide semiconductor whose electroncarrier concentration is less than 10¹⁸/cm³ and which contains In, Ga,and Zn is used. Note that in Patent Document 2, the ratio of In:Ga:Znequals to 1:1:m (m<6) in the amorphous oxide semiconductor.

Further, in Patent Document 3, a field-effect transistor is disclosed inwhich an amorphous oxide semiconductor including a microcrystal is usedfor an active layer.

REFERENCES

[Patent Document 1]

Japanese Published Patent Application No. 2004-103957

[Patent Document 2]

PCT International Publication No. 05/088726

[Patent Document 3]

Japanese Published Patent Application No. 2006-165529

SUMMARY OF THE INVENTION

In Patent Document 3, a disclosure is given in which a composition in acrystal state is InGaO₃(ZnO)_(m) (m is an integer number of less than6). Further, in Example 1 of Patent Document 3, a case of InGaO₃(ZnO)₄is disclosed. However, in the real condition, adequate characteristicshave not been obtained even when such oxide semiconductors are used.

In view of the foregoing problems, it is an object to provide an oxidesemiconductor which is suitable for use in a semiconductor device.Alternatively, it is another object to provide a semiconductor deviceusing the oxide semiconductor.

In the invention to be disclosed, a semiconductor device is manufacturedsuch that crystal grains represented by InGaO₃(ZnO)_(m) (m=1) areincluded in an amorphous structure represented by InGaO₃(ZnO)_(m) (m>0).Specifics are described below.

One embodiment of the invention to be disclosed is a semiconductordevice including an In—Ga—Zn—O based oxide semiconductor layer used fora channel formation region of a transistor. In the semiconductor device,the In—Ga—Zn—O based oxide semiconductor layer has a structure in whichcrystal grains represented by InGaO₃(ZnO)_(m) (m=1) are included in anamorphous structure represented by InGaO₃(ZnO)_(m) (m>0).

In the above, a Zn content (atomic %) in the In—Ga—Zn—O based oxidesemiconductor layer is preferably less than an In content (atomic %) andless than a Ga content (atomic %). Further, the oxide semiconductorlayer is preferably formed by a sputtering method using a target inwhich a Zn content (atomic %) is less than or equal to an In content(atomic %) and less than or equal to a Ga content (atomic %). Further,in the above, it is preferable that only crystal grains each having astructure represented by InGaO₃(ZnO)_(m) (m=1) be used. However, underthe conditions where the proportion of the structure represented byInGaO₃(ZnO)_(m) (m=1) in crystal grains is 80 vol % or more, givencharacteristics can be obtained.

Another embodiment of the invention to be disclosed is a method formanufacturing a semiconductor device including the steps of: forming anIn—Ga—Zn—O based oxide semiconductor layer having an amorphous structureover a substrate by a sputtering method; and subjecting the oxidesemiconductor layer to heat treatment to form an oxide semiconductorlayer including crystal grains represented by InGaO₃(ZnO)_(m) (m=1) inan amorphous structure represented by InGaO₃(ZnO)_(m) (m>0). In themethod for manufacturing a semiconductor device, the oxide semiconductorlayer including crystal grains is used for a channel formation region ofa transistor.

In the above, the In—Ga—Zn—O based oxide semiconductor layer having anamorphous structure is preferably formed such that a Zn content (atomic%) is less than an In content (atomic %) and less than a Ga content(atomic %). Further, the In—Ga—Zn—O based oxide semiconductor layerhaving an amorphous structure is preferably formed by a sputteringmethod using a target in which a Zn content (atomic %) is less than orequal to an In content (atomic %) and less than or equal to a Ga content(atomic %). Furthermore, the heat treatment is preferably performed at350° C. or more.

Note that in this specification, a semiconductor device means any devicewhich can function by utilizing semiconductor characteristics; and adisplay device, a semiconductor circuit, and an electronic device areall included in the semiconductor devices.

Crystal grains represented by InGaO₃(ZnO)_(m) (m=1) are included in anamorphous structure represented by InGaO₃(ZnO)_(m) (m>0), whereby theelectrical characteristics of an oxide semiconductor can be improved.Further, by using the oxide semiconductor, an excellent semiconductordevice can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are explanatory views of a manufacturing process of anoxide semiconductor layer relating to Embodiment 1.

FIG. 2 is a table showing the results of analyzing the compositions ofnon-single-crystal semiconductor layers.

FIG. 3 is a table showing the results of analyzing the compositions ofnon-single-crystal semiconductor layers.

FIGS. 4A and 4B are bright-field-STEM images of oxide semiconductorlayers.

FIGS. 5A and 5B are a bright-field-STEM image and a HAADF-STEM image,respectively.

FIGS. 6A and 6B are an enlarged image of a crystal structure and a modelview of the crystal structure, respectively.

FIGS. 7A and 7B are HAADF-STEM images of an oxide semiconductor layer (atarget B).

FIG. 8 is a view illustrating a crystal structure of an oxidesemiconductor (InGaZnO₄).

FIGS. 9A and 9B are views illustrating the distributions of Ga atoms andZn atoms in (Ga, Zn)O₂ layers.

FIG. 10 is a table showing the result of comparing the energy of thedistribution patterns of elements.

FIG. 11 is a view illustrating the result of calculating DOS and PDOS ofelectrons.

FIGS. 12A and 12B are distribution maps of electrons at the bottom ofthe conduction band.

FIG. 13 is a table showing the result of calculating geometrical optimalvalues of E_(VO).

FIGS. 14A to 14D are views illustrating specific combinations (thedistributions of atoms) relating to calculation.

FIG. 15 is a table showing the energy of each combination.

FIG. 16 is a model view of the most possible distribution.

FIGS. 17A to 17E are explanatory views of a manufacturing process of asemiconductor device relating to Embodiment 2.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to drawings.However, the present invention is not limited to the description ofembodiments given below, and it will be obvious to those skilled in theart that various changes and modifications in modes and details thereofcan be made without departing from the purpose of the present invention.Further, structures relating to different embodiments can be used incombination as appropriate. Note that the same portion or a portionhaving the same function is denoted by the same reference numeral instructures of the present invention described below, and the repetitiveexplanation thereof is omitted.

Embodiment 1

In this embodiment, a method for manufacturing an oxide semiconductorlayer in which crystal grains represented by InGaO₃(ZnO)_(m) (m=1) areincluded in an amorphous structure represented by InGaO₃(ZnO)_(m) (m>0)(the oxide semiconductor layer is referred to as an In—Ga—Zn—O basedoxide semiconductor layer) is described with reference to drawings.

First, an In—Ga—Zn—O based non-single-crystal semiconductor layer 102 isformed over a formation surface (here, over a substrate 100) (see FIG.1A). For example, the non-single-crystal semiconductor layer 102 can beformed by a sputtering method using an oxide semiconductor targetincluding indium (In), gallium (Ga), and zinc (Zn). For example, thefollowing conditions may be employed for the sputtering: a distancebetween the substrate 100 and the oxide semiconductor target is 30 mm to500 mm; the pressure is 0.1 Pa to 2.0 Pa; the DC power source is 0.2 kWto 5.0 kW (when the target with a size of 8-inch in diameter is used);and the atmosphere is an argon atmosphere, an oxygen atmosphere, or amixed atmosphere of argon and oxygen.

Here, the non-single-crystal semiconductor layer 102 is formed under thefollowing conditions: the composition of the oxide semiconductor targetis set such that the ratio of In₂O₃:Ga₂O₃:ZnO equals to 1:1:1; thedistance between the substrate 100 and the oxide semiconductor target is170 mm; the pressure is 0.4 Pa; the DC power is 0.5 kW; the flow rate ofan argon gas is 10 sccm; and the flow rate of an oxygen gas is 5 sccm.

Then, the composition of a sample manufactured by the above-describedmethod is analyzed by inductively coupled plasma mass spectrometry(ICP-MS). The composition of the non-single-crystal semiconductor layer102 formed under the conditions where the flow rate of an argon gas is10 sccm and the flow rate of an oxygen gas is 5 sccm isInGa_(0.94)Zn_(0.40)O_(3.31). FIG. 2 shows the result of theabove-described analysis and the result of analyzing anon-single-crystal semiconductor layer formed under the conditions wherethe flow rate of an argon gas is 40 sccm and the flow rate of an oxygengas is 0 sccm.

Further, FIG. 3 shows the results of analyses using Rutherfordbackscattering spectrometry (RBS) as an analysis method.

FIG. 2 and FIG. 3 show that Ga and Zn contents in a non-single-crystalsemiconductor layer tend to be lower than that in the composition of thetarget. In addition, the results of analyzing the non-single-crystalsemiconductor layers are different depending on manufacturingconditions, an analysis method, and the like.

Next, the non-single-crystal semiconductor layer is subjected to heattreatment at a temperature of 350° C. to 800° C. (preferably, 500° C. to750° C.) for about 10 minutes to 200 minutes, whereby an oxidesemiconductor layer 106 including crystal grains 104 in an amorphousstructure is obtained (see FIG. 1B). A thin film transistor manufacturedusing the oxide semiconductor layer 106 has good electricalcharacteristics: the on/off ratio is 10⁹ or higher and mobility is 10cm²/Vs or higher at a gate voltage of ±20 V. Note that, here, the heattreatment is performed at a temperature of 700° C. for one hour.

After the heat treatment is performed, the structure of the oxidesemiconductor layer 106 is analyzed. Specifically, the cross section ofthe sample is observed with respect to a scanning transmission electronmicroscope (STEM) image.

FIGS. 4A and 4B are bright-field-STEM images of the samples. FIG. 4A isa STEM image of a sample manufactured using a target ofIn₂O₃:Ga₂O₃:ZnO=1:1:1 (In:Ga:Zn=1:1:0.5) (hereinafter, a target A). FIG.4B is a STEM image of a sample manufactured using a target ofIn₂O₃:Ga₂O₃:ZnO=1:1:2 (In:Ga:Zn=1:1:1) (hereinafter, a target B) insteadof the target A for comparison with the above sample.

FIG. 4A shows that the oxide semiconductor layer 106 manufactured by theabove method has a structure including the crystal grains 104 in theamorphous structure.

Note that as seen from the comparison between FIG. 4A and FIG. 4B, inthe case where a Zn content in a target is low, the speed of crystalgrowth is lower than that in the case where a Zn content in a target ishigh. By utilizing this fact, the controllability of crystal growth canbe improved. For example, the Zn content (atomic %) in thenon-single-crystal semiconductor layer 102 is made lower than the Incontent (atomic %) and lower than the Ga content (atomic %), whereby theoxide semiconductor layer 106 with good properties can be formed withgood controllability. When the speed of crystal growth is valued, thezinc content in the non-single-crystal semiconductor layer 102 may bemade high.

Next, a minuter region of the sample is observed by a STEM. FIGS. 5A and5B are STEM images of the sample manufactured using the target A. FIG.5A is a bright-field-STEM image, and FIG. 5B is a high-angle annulardark field (HAADF)-STEM image. In FIG. 5A, a regular structure can beseen; however, it is difficult to locate each atom, and in addition,crystal orientation cannot be detected. On the other hand, in FIG. 5B,the positions of white dots corresponding to each atom can be preciselydetected. Further, as can be seen from FIG. 5B, an amorphous structureis left in a lower right region of FIG. 5B.

In a HAADF-STEM image, a contrast proportional to the square of anatomic number is obtained; therefore, the brighter dot indicates theheavier atom. In FIG. 5B, brighter dots represent In and darker dotsrepresent Ga or Zn.

Next, the above crystal structure is considered with reference to FIGS.6A and 6B. Here, FIG. 6A is an enlarged view (an enlarged image) of FIG.5B. FIG. 6B is a model view of the crystal structure of InGaZnO₄(corresponding to InGaO₃(ZnO)_(m), where m is 1) which is seen from a[100] direction. A comparison between FIGS. 6A and 6B shows that crystalgrains in the sample manufactured using the target A has a crystalstructure of InGaO₃(ZnO)₁. Note that it is likely that In and Gacontribute to electrical conduction in an In—Ga—Zn—O based oxidesemiconductor. Therefore, in order to maintain good electricalcharacteristics, the higher the proportion of a crystal structure with ahigh proportion of In and Ga is, in other words, the higher theproportion of the crystal structure of InGaO₃(ZnO)_(m) (m=1) is, themore preferable.

FIGS. 7A and 7B are HAADF-STEM images of a sample manufactured using thetarget B. In FIGS. 7A and 7B, bright dots are regularly present so as toform lines. The distances between lines of the bright dots areapproximately 0.9 nm, approximately 1.15 nm, and approximately 1.4 nm,which correspond to the distances between indium atoms in the crystalstructures of InGaO₃(ZnO)_(m) where m=1, m=2, and m=3, respectively.That is, it can be said that the sample manufactured using the target Bhas crystals of a plurality of compositions, i.e., crystals ofInGaO₃(ZnO)_(m) where m is at least 1 to 3.

As described above, in an In—Ga—Zn—O based oxide semiconductor, In andGa contribute to electrical conduction. Therefore, under the conditionswhere the proportion of In and Ga is low (that is, under the conditionwhere m is large), the electrical characteristics get worse. Thus, bymaking the proportion of the structure of InGaO₃(ZnO)_(m) (m=1) high,the electrical characteristics can be maintained in a good state. Inspecific, the proportion of the crystal structure of InGaO₃(ZnO)_(m)(m=1) in the whole crystal structure is preferably 80 volume % orhigher, more preferably 90 volume % or higher.

As one method for making the proportion of the crystal structure ofInGaO₃(ZnO)_(m) (m=1) high, there is a method for forming thenon-single-crystal semiconductor layer 102 with a low Zn content byusing a target with a low Zn content. For example, a Zn content (atomic%) in the non-single-crystal semiconductor layer 102 may be lower thanan In content (atomic %) and lower than a Ga content (atomic %) in thenon-single-crystal semiconductor layer 102. A Zn content in thenon-single-crystal semiconductor layer 102 is made low, whereby thecrystal structure with good electrical characteristics can be obtained.

The electronic structure of an In—Ga—Zn—O based oxide semiconductor hasnot been sufficiently elucidated, and it is likely that the elucidationof the electronic structure leads to understanding of electricalcharacteristics of an oxide semiconductor. Thus, the result ofcalculating an In—Ga—Zn—O based oxide semiconductor by the firstprinciple calculation and the consideration thereof are described below.Note that although the result given below is obtained by calculating thecrystal structure, the result can be similarly adapted to an amorphousstructure including crystal grains.

FIG. 8 illustrates the crystal structure of an In—Ga—Zn—O based oxidesemiconductor (InGaZnO₄) which is obtained by calculation. The unit cellof the crystal structure of InGaZnO₄ includes 21 atoms. However, here,the calculation is performed with respect to an enlarged unit including84 atoms (2×2×1) in order to examine the distribution of Ga atoms.

The calculation is performed using CASTEP. CASTEP is a first principlecalculation program based on the density functional theory (DFT) and theplane wave pseudo-potential method. For exchange correlation functional,generalized-gradient approximation (GGA) and Perdew-Burke-Ernzerhof(PBE) are selected here. The cut-off energy is set to 500 eV, and theK-point grid of 3×3×1 is used.

FIG. 8 shows that the unit cell includes two (Ga, Zn)O₂ layers and oneInO₂ layer. Note that here, the crystal structure with repeated unitcells including specified atom distribution is assumed for simplicity.

FIGS. 9A and 9B show the distributions of Ga atoms and Zn atoms in two(Ga, Zn)O₂ layers in unit cells. FIGS. 9A and 9B show structures of 2×2times unit cells, i.e., structures each including 4 unit cells. Further,heavy lines in FIGS. 9A and 9B represent a unit cell. FIG. 9A shows thecase where two Ga atoms are present in each of an upper and a lowerlayer, and FIG. 9B shows the case where one Ga atom is present in anupper layer and three Ga atoms are present in a lower layer.

In the case shown in FIG. 9A, Ga atoms in each layer are distributed inthe form of stripes. That is, Ga atoms in each layer are distributed inthe form of parallel lines.

Considering the combination of the upper and the lower layers, thedistributions of Ga atoms are classified into two patterns: a paralleldistribution and a cross distribution. The parallel distribution refersto the distribution where Ga lines in the upper layer are in parallel toGa lines in the lower layer, and the cross distribution refers to thedistribution where Ga lines in the upper layer cross Ga lines in thelower layer. In the case of the parallel distribution, there are twopossible combinations such as U1+L1 and U1+L4. In the case of the crossdistribution, the distribution has a rotationally symmetrical relation;therefore, only one combination like U1+L2 is considered, for example.That is, there are three combinations in total in the case shown in FIG.9A.

In the case shown in FIG. 9B, as combinations of the upper and the lowerlayers, there are two possible combinations such as U7+L7 and U7+L10.Note that in the case where all Ga atoms enter either the upper layer orthe lower layer, there is one combination of the upper and the lowerlayers (U11+L11, not shown). Therefore, for the Ga distribution, sixcombinations may be considered in total.

FIG. 10 shows the result of comparing the energy of the six distributionpatterns. InGaZnO₄ has the lowest energy in the case where two Ga atomsare present in each of the upper and the lower layer, more specifically,in the case of the structure like U1+L1.

The electronic structure of this structure (the structure having thelowest energy) is calculated in more detail. FIG. 11 shows the result ofcalculating density of state (DOS) and projected density of state (PDOS)of electrons in the above structure. According to FIG. 11, Ga atoms arethe most dominant, followed by In atoms.

Next, the existing probability of electron |Ψ|² at the bottom of theconduction band is calculated from an orbital function Ψ at the bottomof the conduction band. FIGS. 12A and 12B show distribution mapsthereof. Here, FIG. 12A shows the existing probability of electrons onthe plane of In (in the InO₂ layer), and FIG. 12B shows the existingprobability of electrons in the (Ga, Zn)O₂ layer. It is interesting thatthe In orbitals are separated.

FIG. 12B shows that the existing probability of electrons in theperiphery of Ga atoms is high and the existing probability of electronsin the periphery of Zn atoms is low. Further, the electrical conductionpath appears to be present not only on the plane of In but also in the(Ga, Zn)O₂ layer. From this, it is likely that Ga largely contributes tothe electrical conduction of InGaZnO₄. It seems that the Ga orbitalaffects the In orbital and contributes to the electrical conduction ofthe (Ga, Zn)O₂ layer.

One of the features of InGaZnO₄ is a wide margin of electricalconductivity. This seems to be due to the probability of formation ofoxygen vacancies (defects) because the electrical conductivities widelyvary depending on the amount of oxygen added in a process. Thus, inorder to clarify this mechanism, the formation energy of oxygenvacancies is calculated.

The energy level of an oxygen vacancy defect which is calculated basedon the density functional theory (DFT) is still being argued. Forexample, a band gap obtained by using a function such as local densityapproximation (LDA) or GGA tends to be smaller than an actualmeasurement value. Since the scaling method is still being discussed,the simple GGA function without the scaling method is used here. It islikely that, by using this, artificial phenomena are eliminated and theessence of phenomena can be grasped.

The formation energy of oxygen vacancies (E_(VO)) is defined as follows:E _(VO) =E(A _(m) O _(n-1))+E(O)−E(A _(m) O _(n))Here, E_(VO) is a half of the energy of an oxygen molecule, andE(A_(m)O_(n-1)) represents the energy of A_(m)O_(n-1) having oxygenvacancies where A represents an optional element.

FIG. 13 shows the result of calculating geometrical optimal values ofE_(VO) in a structure having oxygen vacancies. Here, as a latticeconstant, a lattice constant of an ideal crystal is used. High level ofE_(VO) means that the density of oxygen vacancies in thermal equilibriumis low. Note that, in FIG. 13, E_(VO) of In₂O₃, E_(VO) of ZnO, andE_(VO) of Ga₂O₃ are also shown. Crystal structures of In₂O₃, ZnO, andGa₂O₃ are bixbyite, wurtzite, β-gallia, respectively.

E_(VO) of InGaZnO₄ varies depending on elements in the periphery ofoxygen vacancies. Specifically, there are three possible structuralmodels as follows. In Model 1, an oxygen vacancy is surrounded by one Znatom and three In atoms. In Model 2, an oxygen vacancy is surrounded byone Ga atom and three In atoms. In Model 3, an oxygen vacancy issurrounded by two Zn atoms and two Ga atoms. FIG. 13 shows that thelevel of E_(VO) of InGaZnO₄ becomes higher as the number of Ga atoms inthe periphery of the oxygen vacancy increases. Further, Ga₂O₃ has thehighest level of E_(VO) in FIG. 13; therefore, it can be said that Gaatoms bond with O atoms tightly.

When InGaZnO₄ is in an amorphous state, there are more possiblestructures in addition to the above three types of models, and thelevels of E_(VO) of the structures are slightly different from eachother. As the proportion of Ga atoms in InGaZnO₄ increases, the densityof oxygen vacancies decreases. As the proportion of Ga atoms in InGaZnO₄decreases, the density of oxygen vacancies increases.

Thus, making the proportion of Ga atoms in the non-single-crystalsemiconductor layer 102 high enables reduction of the density of oxygenvacancies. That is, an In—Ga—Zn—O based oxide semiconductor with goodelectrical characteristics can be obtained. Considering the existence ofIn contributing to electrical conduction, it is not preferable to reducethe proportion of In. Therefore, the proportion of Zn in thenon-single-crystal semiconductor layer 102 is preferably made low. Forexample, the Zn content (atomic %) in the non-single-crystalsemiconductor layer 102 may be lower than the In content (atomic %) andlower than the Ga content (atomic %). By making the Zn content in thenon-single-crystal semiconductor layer 102 low in this manner, an oxidesemiconductor layer with good electrical characteristics can beobtained.

According to this embodiment, a high-performance oxide semiconductorlayer can be provided. This embodiment can be implemented in combinationwith any of other embodiments as appropriate.

Embodiment 2

In this embodiment, the result of promoting the consideration of anIn—Ga—Zn—O based oxide semiconductor layer in Embodiment 1 is describedwith reference to drawings.

It is revealed in Embodiment 1 that two (Ga, Zn)O₂ layers and one InO₂layer are included in a unit cell of a crystal structure of InGaZnO₄. Inresponse to this, the distribution of Ga atoms and Zn atoms is studiedin more detail in this embodiment. In specific, some combinations of anupper layer and a lower layer are given as shown in FIGS. 9A and 9B, andthe relation between the distribution of Ga atoms and Zn atoms in two(Ga, Zn)O₂ layers and the energy is calculated and considered.

FIGS. 14A to 14D illustrate specific combinations (the distributions ofatoms) which are subjected to calculation. In this embodiment, thesecombinations are selected with a focus on the number of nearest neighboratoms of the same kind. For example, in a combination shown in FIG. 14A,Ga atoms and Zn atoms are distributed in the upper layer and in thelower layer, respectively, such that the Ga atoms and the Zn atoms areseparated from each other, whereby the number of nearest neighbor atomsof the same kind is made zero. In a combination shown in FIG. 14B, twoatoms of the same kind are present in nearest neighbor positions. In acombination shown in FIG. 14C, one and a half atoms of the same kind arepresent in nearest neighbor positions. In a combination shown in FIG.14D, one atom of the same kind is present in a nearest neighborposition. The calculation is performed under the same conditions asEmbodiment 1.

FIG. 15 shows the calculation result. In FIG. 15, the structure with thelowest energy is regarded as an origin point (the energy is regarded as0 eV) to show the energy of each structure.

Although the distributions examined in this embodiment are only part ofa number of distributions, the tendency of the distribution of Ga atomsand Zn atoms can be read from FIG. 15. It is likely that the resultshown in FIG. 15 show that the structure becomes stable in terms ofenergy as the degree of aggregation of atoms of the same kind becomeslower. Thus, it can be concluded that, in the crystal structure ofInGaZnO₄, Ga atoms and Zn atoms are not present in an aggregation stateas GaO or ZnO but are distributed to be mixed each other in an atomstate. FIG. 16 shows a model view of the most possible distribution(corresponding to the distribution (d) in FIG. 15).

This embodiment can be implemented in combination with any of otherembodiments as appropriate.

Embodiment 3

In this embodiment, an example of a manufacturing process of asemiconductor device using an oxide semiconductor layer manufactured bythe method described in Embodiment 1 or the like is described withreference to FIGS. 17A to 17E. Note that in this embodiment, thedetailed description of the content which is the same or substantiallythe same as in Embodiment 1 or the like is omitted.

First, a gate electrode 202 is formed over a substrate 200 having aninsulating surface, and a gate insulating layer 204 is formed over thegate electrode 202; then, an oxide semiconductor layer 206 and an oxidesemiconductor layer 207 are stacked (see FIG. 17A).

As the substrate 200 having an insulating surface, for example, a glasssubstrate having a visible light transmitting property, which is usedfor a liquid crystal display device or the like, can be used. The glasssubstrate is preferably an alkali-free glass substrate. As analkali-free glass substrate, for example, a glass material such asaluminosilicate glass, aluminoborosilicate glass, or barium borosilicateglass is used. In addition, as the substrate 200 having an insulatingsurface, an insulating substrate which is formed using an insulator,such as a resin substrate, a ceramic substrate, a quartz substrate, or asapphire substrate; a semiconductor substrate which is formed using asemiconductor material such as silicon and whose surface is covered withan insulating material; a conductive substrate which is formed using aconductor such as metal or stainless steel and whose surface is coveredwith an insulating material; or the like can also be used. From theviewpoint of increasing the area of a semiconductor device,specifically, a glass substrate is preferably used. Further, thesubstrate 200 having an insulating surface preferably has a given heatresistant property.

A conductive layer is formed over an entire surface of the substrate 200and then the conductive layer is selectively etched using a resist maskformed by a photolithography method, whereby the gate electrode 202 canbe formed. At this time, in order to improve the coverage of the gateelectrode 202 with the gate insulating layer 204 formed later andpreventing disconnection, the gate electrode 202 is preferably etched sothat end portions thereof have a tapered shape. Note that the gateelectrode 202 includes an electrode and a wiring, such as a gate wiring,formed using the conductive layer.

The gate electrode 202 is desirably formed using a low resistanceconductive material such as aluminum (Al) or copper (Cu). Note that inthe case where aluminum is used for the wiring and the electrode,aluminum has disadvantages such as low heat resistance and a tendency tobe corroded when aluminum is used by itself; therefore, aluminum ispreferably used in combination with a heat-resistant conductivematerial.

As the heat-resistant conductive material, an element selected fromtitanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium(Cr), neodymium (Nd), and scandium (Sc), an alloy including any of theabove elements as its component, an alloy including a combination of anyof these elements, a nitride including any of the above elements as itscomponent, or the like can be used. A film formed using such aheat-resistant conductive material and aluminum (or copper) are stacked,so that the wiring and the electrode can be formed.

The gate insulating layer 204 can be formed using a silicon oxide film,a silicon oxynitride film, a silicon nitride film, a silicon nitrideoxide film, an aluminum oxide film, a tantalum oxide film, or the like.Alternatively, the gate insulating layer 204 may be formed using a stackof these films. The film can be formed to a thickness of 20 nm to 250 nminclusive by a sputtering method or the like. For example, as the gateinsulating layer 204, a silicon oxide film is formed to a thickness of100 nm by a sputtering method. Note that the gate insulating layer 204may serve as a gate insulating layer of a transistor, and themanufacturing method, the thickness, and the like of the gate insulatinglayer 204 are not construed as being limited to the range of the numericvalues described above.

Note that before the oxide semiconductor layer 206 is formed over thegate insulating layer 204, a surface of the gate insulating layer 204may be subjected to plasma treatment. Dust attaching to the surface ofthe gate insulating layer 204 can be removed by the plasma treatment.

An inert gas such as an argon (Ar) gas is introduced into a vacuumchamber, and a bias voltage is applied to a processing object (here, thesubstrate 200 over which the gate insulating layer 204 is formed) sothat a plasma state is generated, whereby the above-described plasmatreatment can be performed. In this case, electrons and cations of Arare present in the plasma and the cations of Ar are accelerated in acathode direction (toward the substrate 200 side). The acceleratedcations of Ar collide with the surface of the gate insulating layer 204,whereby the surface of the gate insulating layer 204 is etched bysputtering so as to be reformed. In place of an argon gas, a helium gasmay be used. Alternatively, the plasma treatment may be performed in anargon atmosphere to which oxygen, hydrogen, nitrogen, and/or the like isadded. Further alternatively, the plasma treatment may be performed inan argon atmosphere to which chlorine (Cl₂), carbon tetrafluoride (CF₄),and/or the like is added. Such plasma treatment described above may alsobe referred to as “reverse sputtering”.

The oxide semiconductor layer 206 can be formed using an In—Ga—Zn—Obased non-single-crystal semiconductor layer. For example, the oxidesemiconductor layer 206 is formed by a sputtering method using an oxidesemiconductor target including indium (In), gallium (Ga), and zinc (Zn)(In₂O₃:Ga₂O₃:ZnO=1:1:1). For the sputtering conditions, Embodiment 1 andthe like can be referred to.

Note that a pulsed DC power source is preferably used in the sputteringbecause dust can be reduced and thickness can be uniformed. In addition,the above plasma treatment is performed and then the oxide semiconductorlayer 206 is formed without being exposed to the air, so that dust ormoisture can be prevented from attaching to the interface between thegate insulating layer 204 and the oxide semiconductor layer 206. Thethickness of the oxide semiconductor layer 206 may be approximately 5 nmto 500 nm.

The oxide semiconductor layer 207 can be formed using an In—Ga—Zn—Obased non-single-crystal semiconductor layer in a manner similar to theoxide semiconductor layer 206. For example, the oxide semiconductorlayer 207 can be formed over the oxide semiconductor layer 206 by asputtering method using an oxide semiconductor target including In, Ga,and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1). At this time, it is preferable that theoxide semiconductor layer 207 be successively formed without exposingthe oxide semiconductor layer 206 to the air. For example, the followingconditions may be employed for the sputtering: the temperature is 20° C.to 100° C.; the pressure is 0.1 Pa to 2.0 Pa; and the DC power source is250 W to 3 kW (when the target with a size of 8-inch in diameter isused). Further, an argon gas may be introduced into the atmosphere.

The oxide semiconductor layer 206 and the oxide semiconductor layer 207are preferably formed under the different film formation conditions. Forexample, in the film formation conditions of the oxide semiconductorlayer 206, the flow rate ratio of an oxygen gas to an argon gas is madehigher than that in the film formation conditions of the oxidesemiconductor layer 207. Specifically, the oxide semiconductor layer 207is formed in a noble gas (such as argon or helium) atmosphere (or anatmosphere containing oxygen at 10% or less and a noble gas at 90% ormore), and the oxide semiconductor layer 206 is formed in an oxygenatmosphere or an atmosphere in which the flow rate of an oxygen gas isequal to or more than the flow rate of a noble gas). Accordingly, theoxide semiconductor layer 207 which is higher in electrical conductivitythan the oxide semiconductor layer 206 can be formed.

As the above sputtering method for forming the oxide semiconductor layer206 and the oxide semiconductor layer 207, an RF sputtering method inwhich a high frequency power source is used as a sputtering powersource, a DC sputtering method, a pulsed DC sputtering method in whichdirect current bias is applied in pulses, or the like can be employed.

Alternatively, a multi-target sputtering apparatus which can be providedwith a plurality of targets formed using different materials may beused. In a multi-target sputtering apparatus, a stack of different filmscan be formed in one chamber, or one film can be formed by sputteringusing plural kinds of materials at the same time in one chamber.Alternatively, a method using a magnetron sputtering apparatus in whicha magnetic field generating system is provided inside the chamber (amagnetron sputtering method), an ECR sputtering method in which plasmagenerated by using a microwave is used, or the like may be employed.Further alternatively, a reactive sputtering method in which a targetsubstance and a sputtering gas component are chemically reacted witheach other to form a compound thereof at the time of film formation, abias sputtering method in which a voltage is applied also to thesubstrate at the time of film formation, or the like may be employed.

Note that in this embodiment, an example of the case where the oxidesemiconductor layer 206 and the oxide semiconductor layer 207 arestacked is described; however, the invention to be disclosed is notlimited thereto. For example, the structure without the oxidesemiconductor layer 207 (the structure in which only the oxidesemiconductor layer 206 is formed) may be used.

Next, a resist mask 208 is formed over the oxide semiconductor layer207. Then, the oxide semiconductor layer 206 and the oxide semiconductorlayer 207 are selectively etched using the resist mask 208, so that anisland-shaped oxide semiconductor layer 210 and an island-shaped oxidesemiconductor layer 211 are formed (see FIG. 17B).

As the above etching, wet etching is preferably employed. Here, theisland-shaped oxide semiconductor layer 210 and the island-shaped oxidesemiconductor layer 211 are formed by removing an unnecessary portion ofthe oxide semiconductor layer 206 and the oxide semiconductor layer 207by wet etching with the use of ITO07N (manufactured by Kanto ChemicalCo., Inc.) or a mixed solution of acetic acid, nitric acid, andphosphoric acid. Note that after the above etching, the resist mask 208is removed. In addition, an etchant for the wet etching is not limitedto the above solution as long as the oxide semiconductor layer 206 andthe oxide semiconductor layer 207 can be etched using the etchant. Ofcourse, dry etching may be used as the above etching.

Next, a conductive layer 212 is formed over the island-shaped oxidesemiconductor layer 211 (see FIG. 17C).

The conductive layer 212 can be formed by a sputtering method, a vacuumevaporation method, or the like, using a metal including an elementselected from aluminum (Al), copper (Cu), titanium (Ti), tantalum (Ta),tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), andscandium (Sc), an alloy including any of the above elements as itscomponent, an alloy including a combination of any of these elements, ora nitride including any of the above elements as its component. Notethat in this embodiment, heat treatment (e.g., heat treatment at 350° C.to 800° C. (preferably, 500° C. to 750° C.)) is performed afterformation of the conductive layer 212; therefore, the conductive layer212 preferably has a given heat resistant property.

For example, the conductive layer 212 can be formed with a single-layerstructure of a titanium film. Alternatively, the conductive layer 212 isformed with a stacked-layer structure. For example, the conductive layer212 can be formed with a stacked-layer structure of an aluminum film anda titanium film. Further alternatively, a three-layer structure of atitanium film, an aluminum film including a neodymium (Al—Nd) film, anda titanium film may be employed. Further alternatively, the conductivelayer 212 may be formed with a single-layer structure of an aluminumfilm including silicon.

Next, a resist mask 214 a, a resist mask 214 b, and a resist mask 214 care formed over the conductive layer 212, and the conductive layer 212is selectively etched so that a conductive layer 216 a, a conductivelayer 216 b, and a conductive layer 218 are formed. In addition, theisland-shaped oxide semiconductor layer 211 is etched so that asemiconductor region 215 a with high conductivity and a semiconductorregion 215 b with high conductivity are formed. Further, part of theisland-shaped oxide semiconductor layer 210 (the part near a surfacethereof) is removed (channel-etched) (see FIG. 17D).

A recessed portion 220 formed by removing the part of the island-shapedoxide semiconductor layer 210 and part of the island-shaped oxidesemiconductor layer 211 corresponds to a region which is between theconductive layer 216 a and the conductive layer 216 b and also betweenthe semiconductor region 215 a with high conductivity and thesemiconductor region 215 b with high conductivity. Therefore, theconductive layer 216 a functions as one of a source electrode and adrain electrode of the transistor, while the conductive layer 216 bfunctions as the other of the source electrode and the drain electrode.As illustrated in FIG. 17D, the recessed portion 220 is formed byremoving the part of the oxide semiconductor layer 210 and the part ofthe island-shaped oxide semiconductor layer 211, whereby the conductivelayer 216 a and the conductive layer 216 b can be electrically insulatedfrom each other without fault. In addition, the conductive layer 218functions as a wiring which electrically connects transistors and thelike.

As the above etching, dry etching is preferably employed. By employingdry etching, a wiring structure or the like can be miniaturized ascompared to the case of using wet etching. In addition, by employing dryetching, etching is performed with high controllability; therefore, theremoval of the part of the island-shaped oxide semiconductor layer 210(the formation of the recessed portion 220) can be performed with highcontrollability. As the gas which can be used for the dry etching, achlorine-based gas such as chlorine (Cl₂), boron chloride (BCl₃),silicon chloride (SiCl₄), or carbon tetrachloride (CCl₄); afluorine-based gas such as carbon tetrafluoride (CF₄), sulfur fluoride(SF₆), nitrogen fluoride (NF₃), or trifluoromethane (CHF₃); hydrogenbromide (HBr); oxygen (O₂); any of these gases to which a noble gas suchas helium (He), or argon (Ar) is added; or the like can be used. Ofcourse, wet etching may be used as the above etching.

In addition, as a material for the conductive layer 212, a materialwhose etching rate is higher than that of the island-shaped oxidesemiconductor layer 210 or the island-shaped oxide semiconductor layer211 is preferably used. This is because when the conductive layer 212,the island-shaped oxide semiconductor layer 210, and the island-shapedoxide semiconductor layer 211 are etched at one time, the etching ratesof the island-shaped oxide semiconductor layer 210 and the island-shapedoxide semiconductor layer 211 are made smaller than that of theconductive layer 212, so that the island-shaped oxide semiconductorlayer 210 can be prevented from being etched excessively.

Note that the resist mask 214 a, the resist mask 214 b, and the resistmask 214 c are removed after the above etching.

After that, heat treatment is performed at a given temperature (forexample, 350° C. to 800° C. (preferably 500° C. to 750° C.)). Note thatin the case where a glass substrate is used as the substrate 200 havingan insulating surface, it is necessary to perform heat treatment at atemperature lower than or equal to the strain point of the glasssubstrate. The heat treatment may be performed in an air atmosphere or anitrogen atmosphere. By the heat treatment, crystal grains of an oxidesemiconductor are grown in the island-shaped oxide semiconductor layer210. Thus, an oxide semiconductor layer including crystal grainsrepresented by InGaO₃(ZnO)_(m) (m=1) in an amorphous structurerepresented by InGaO₃(ZnO)_(m) (m>0) (an In—Ga—Zn—O based semiconductorlayer) can be obtained.

An oxide semiconductor with an amorphous structure easily becomes anoxide semiconductor with a crystal structure due to heat or the like.Therefore, in the case where the proportion of the amorphous structureis high, a transistor tends to reduce its reliability. From theviewpoint of increasing the reliability, the heat treatment is performedso that the proportion of the amorphous structure is 90 volume % or less(preferably 80 volume % or less, more preferably 60 volume % or less).

Although heat treatment time can be changed in relation with heattreatment temperature as appropriate, for example, the heat treatmenttime can be approximately 0.5 hour to 2 hours at a temperature of 700°C. Further, since a temperature suitable for the heat treatment dependson the composition of an intended oxide semiconductor, there is noparticular limitation on the heat treatment temperature as long as adesired oxide semiconductor layer can be obtained.

Note that, for the above heat treatment, a diffusion furnace, a heatingfurnace such as a resistance heating furnace, a rapid thermal annealing(RTA) apparatus, a microwave heating apparatus, or the like can be used.The irradiation with light having a wavelength which is absorbed by anoxide semiconductor (an electromagnetic wave) may be performed insteadof the heat treatment. That is, the structure including crystal grainsin an amorphous structure may be realized by irradiation with light (anelectromagnetic wave). In this case, a laser capable of oscillatinglight having a short wavelength, an ultraviolet lamp, or the like may beused as a light source.

As described above, an oxide semiconductor layer including crystalgrains represented by InGaO₃(ZnO)_(m) (m=1) in an amorphous structurerepresented by InGaO₃(ZnO)_(m) (m>0) is used for a channel formationregion of a transistor, whereby a high-performance semiconductor devicecan be provided.

Here, in order to realize an oxide semiconductor layer with goodelectrical characteristics, a Zn content (atomic %) in an oxidesemiconductor is preferably set lower than an In content (atomic %) andlower than a Ga content (atomic %), for example. With such acomposition, an oxide semiconductor layer with good electricalcharacteristics can be obtained.

Note that an oxide semiconductor layer with a Zn content (atomic %)lower than an In content (atomic %) and lower than a Ga content (atomic%) as described above can be formed by a sputtering method using atarget with a composition close to an intended composition. In thiscase, considering FIGS. 2 and 3, a decreasing proportion of Zn in aformed oxide semiconductor layer as compared to the proportion of Zn ina composition of the target is larger than those of In and Ga.Therefore, a target with a Zn content (atomic %) equal to an In or Gacontent (atomic %) may be used in order to form an oxide semiconductorlayer with a Zn content (atomic %) lower than an In or Ga content(atomic %), for example. That is, a target with a Zn content (atomic %)lower than or equal to an In or Ga content (atomic %) may be used.

Here, in this embodiment, an example of the case where the heattreatment is performed after the island-shaped semiconductor layer 210is formed is given. However, there is no particular limitation on thetiming of performing the heat treatment as long as the heat treatment isperformed after the island-shaped semiconductor layer 206 is formed.Further, the heat treatment is not needed if a structure including aplurality of crystal grains in an amorphous structure (a structure inwhich a plurality of crystal grains are dispersed in an amorphousstructure) is obtained at the stage of film formation.

Note that the recessed portion 220 of the exposed part of theisland-shaped oxide semiconductor layer 210 may be subjected to oxygenradical treatment. By performing the oxygen radical treatment, the thinfilm transistor in which the island-shaped oxide semiconductor layer 210is a channel formation region can be easily normally off. Further, byperforming radical treatment, damage to the island-shaped oxidesemiconductor layer 210 due to etching can be repaired. The radicaltreatment is preferably performed under an O₂ atmosphere; an N₂Oatmosphere; an N₂, He, or Ar atmosphere including oxygen; or the like.In addition, radical treatment may be performed under an atmosphere inwhich Cl₂ and/or CF₄ is added to the above atmosphere. Note that radicaltreatment is preferably performed without application of a bias voltageon the substrate 200 side.

Next, a protective insulating layer 222 is formed so as to cover thethin film transistor 250 including the gate electrode 202, theisland-shaped oxide semiconductor layer 210, the semiconductor region215 a with high conductivity, the semiconductor region 215 b with highconductivity, the conductive layer 216 a, the conductive layer 216 b,and the like (see FIG. 17E). The protective insulating layer 222 may beformed by a sputtering method or the like using a material includingsilicon nitride, silicon oxide, silicon nitride oxide, siliconoxynitride, aluminum oxide, tantalum oxide, or the like.

After that, various kinds of electrodes and wirings are formed. Thus,the semiconductor device is completed.

According to this embodiment, a high-performance semiconductor devicecan be provided. Note that this embodiment can be implemented incombination with any of other embodiments as appropriate.

This application is based on Japanese Patent Application serial no.2008-296901 filed with Japan Patent Office on Nov. 20, 2008, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: an oxidesemiconductor layer comprising a channel formation region, wherein theoxide semiconductor layer comprises crystals represented by InGaZnO₄,wherein the oxide semiconductor layer comprises In, Ga, and Zn, whereina Zn content by atomic percent in the oxide semiconductor layer is lessthan an In content by atomic percent in the oxide semiconductor layer,wherein the Zn content by atomic percent in the oxide semiconductorlayer is less than a Ga content by atomic percent in the oxidesemiconductor layer, and wherein in the oxide semiconductor layer, aproportion of the crystals represented by InGaZnO₄ to a whole ofcrystals is 80 volume % or more.
 2. The semiconductor device accordingto claim 1, wherein the oxide semiconductor layer is formed over and incontact with an insulating layer.
 3. The semiconductor device accordingto claim 2, wherein the insulating layer comprises at least one filmselected from a silicon oxide film, a silicon oxynitride film, a siliconnitride film, and a silicon nitride oxide film.
 4. The semiconductordevice according to claim 1, wherein the oxide semiconductor layer isformed using a target of In:Ga:Zn=2:2:1 by sputtering method.
 5. Thesemiconductor device according to claim 1, wherein the semiconductordevice is a transistor, and wherein an on/off ratio of the transistor is10⁹ or higher and mobility of the semiconductor device is 10 cm²Ns orhigher at a gate voltage of ±20 V.
 6. A semiconductor device comprising:a first oxide semiconductor layer comprising a channel formation region;and a second oxide semiconductor layer over the first oxidesemiconductor layer, wherein the first oxide semiconductor layercomprises crystals represented by InGaZnO₄, wherein the first oxidesemiconductor layer comprises In, Ga, and Zn, wherein a Zn content byatomic percent in the first oxide semiconductor layer is less than an Incontent by atomic percent in the first oxide semiconductor layer,wherein the Zn content by atomic percent in the first oxidesemiconductor layer is less than a Ga content by atomic percent in thefirst oxide semiconductor layer, and wherein a proportion of thecrystals represented by InGaZnO₄ to a whole of crystals is 80 volume %or more in the first oxide semiconductor layer.
 7. The semiconductordevice according to claim 6, wherein a conductivity of the first oxidesemiconductor layer is different from a conductivity of the second oxidesemiconductor layer.
 8. The semiconductor device according to claim 7,wherein the first oxide semiconductor layer is formed over and incontact with an insulating layer.
 9. The semiconductor device accordingto claim 8, wherein the insulating layer comprises at least one filmselected from a silicon oxide film, a silicon oxynitride film, a siliconnitride film, and a silicon nitride oxide film.
 10. The semiconductordevice according to claim 6, wherein the first oxide semiconductor layeris formed using a target of In:Ga:Zn=2:2:1 by sputtering method.
 11. Thesemiconductor device according to claim 6, wherein the semiconductordevice is a transistor, and wherein an on/off ratio of the transistor is10⁹ or higher and mobility of the semiconductor device is 10 cm²/Vs orhigher at a gate voltage of ±20 V.